It was Mr. Hansen who noticed the shaking at first. Although he didn't know it, it was thanks to his innate magical energy that he was able to feel the shaking caused by Rita when it was still much smaller than what a normal person would feel. It was thanks to this that the history teacher was able to yell one very important thing in time: 

"Earthquake! Everyone get to cover!" 

The students and faculty inside the school did not seem to understand why he was yelling such nonsense right until the moment the shaking became strong enough for everyone else to notice it too. Quickly, the teachers, including Leonard, ushered the students under desks, under tables, under benches, whatever sort of cover they could find in order to protect themselves from what they thought was just a normal earthquake - something that was quite rare in these parts. 

And he had hoped for such a normal day too. .